1. Field of the Invention
This invention relates to water-soluble polymeric dispersing and thickening agents of lower alkyl vinyl ether and maleic anhydride, and, more particularly, to crosslinked terpolymers of high salt tolerance.
2. Description of the Prior Art
Water-soluble polymeric dispersing and thickening agents of lower alkyl vinyl ether and maleic anhydride optionally including .alpha.-olefin monomer have been used in many latex and other formulations. However, high concentrations of these relatively low molecular weight polymers are required to provide improved results. In order to enhance their molecular weight and efficient use as thickeners, the polymers may be crosslinked with a polyfunctional compound, e.g. a C.sub.8 to C.sub.14 diene. It has been found that this innovation is not entirely satisfactory for, although crosslinking initially increases the viscosity, the salt tolerance of these crosslinked polymers is extremely low. Thus, upon addition to compositions containing a substantial amount of salt component, such as the ammonium and metal sulfates, citrates, chlorides, EDTA, etc. normally present in a concentration of about 1% in most cosmetic formulations, the polymer loses between 80 to 95% of its initial viscosity and fails to provide the desired thickening affect. Additionally, the salt containing polymeric compositions do not possess sufficient stability for storage and are subject to precipitation and phase separation. These affects are particularly troublesome when calcium or sodium chlorides are present in the composition.
Non-crosslinked alkyl vinyl ether, maleic anhydride and higher alkyl methacrylate terpolymers also have been employed as latex thickeners as shown in U.S. Pat. No. 3,436,378. However, these terpolymers are also subject to phase separation and, being of relatively low molecular weight and viscosity, do not provide a high degree of thickening. Additionally, these terpolymers do not form hydrogels.
Kwak, in U.S. Pat. No. 5,362,789, described a crosslinked terpolymer of high salt tolerance of a C.sub.1 -C.sub.4 alkyl vinyl ether, maleic anhydride and a C.sub.10 -C.sub.20 alkyl (meth)acrylate as the hydrophobic monomer.
Accordingly, it is an object of this invention to provide an improved crosslinked terpolymer of high salt tolerance, whose hydrogel retains up to 100% of its initial viscosity in salt-containing compositions, has a low residual monomer level of &lt;1000 ppm each, and enhanced use characteristics.